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Nymagee, Hera, Federation |
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New South Wales, NSW, Australia |
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Zn Pb Au Cu Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Nymagee, Hera and Federation deposits are located within splays west of the NNW-SSE trending Rookery Fault Zone, ~96 km SSE of Cobar, and ~490 km ENE of Sydney in New South Wales, Australia. Hera and Federation are 4.5 km SSE and 15 km south of Nymagee respectively.
Copper mineralisation was first discovered by two stockmen, Manly and Bryson, near Nymagee in September 1876. With the assistance of Russell Barton, who was also a major investor in the Cobar Copper mine to the north, the Nymagee Copper Mining Company Ltd was formed in March 1880. Barton subsequently purchased the company. Shaft sinking commenced almost immediately. Early development on the main lode encountered copper carbonates and then chalcocite to a depth of ~50 m and width of up to 12 m. Subsequently yellow sulphides were encountered at greater depths by October 1880 with reduced thicknesses of nearer 2 m. Smelting commenced in early December 1880. Operations continued until 1887 with 82 894 t of ore had been smelted to produce 10 566 t of copper metal. Production continued at a reduced rate until late 1892 when the mine was closed due to the continuing low copper price and the company was liquidated. Between 1896 and 1906, the mine was purchased by the Great Cobar Mining Syndicate and mining resumed in 1897. In December 1906, the Great Cobar Syndicate sold the Nymagee operation to a group of British investors who formed the Nymagee Copper Limited Company. The mine was closed and the smelting infrastructure rebuilt to take advantage of more modern processes. Smelting recommenced in October 1907, but the mine was closed in 1908. A re-evaluation of the mine in 1908 estimated a remaining resource of 0.213 Mt of contained copper at a grade averaging 3.8% Cu. However, as the NSW government could not be convinced to build a railway line to Nymagee, at the then ruling metal prices and cost of alternative transport, the deposit was considered uneconomic. No more work was undertaken until late1912 when there was a rise in the price of copper and another British funded company, the Mouramba Copper Mines Ltd purchased the mine. They upgraded the smelting facilities and operated for 4 months from October 1913, but closed again. When the copper price rebounded, mining recommenced in 1915, but closed yet again in November 2016. The Nymagee mine was finally closed at the end of 1917. Cumulative production from 1881 to 1917 had grown to 25 198 t of Cu metal from ore averaging 10% Cu to 1896, then to 4% and finally 2% Cu and the mines had been worked to a depth of 244 m. This production came from 0.422 Mt @ 5.8% Cu. Various companies explored the area in the 1960s and 1970s, with Cyprus Mines Corporation in 1973 defining a remnant pillar resource at the Nymagee mine of 0.821 Mt @ 2.3% Cu and a total resource in the Pb-Zn lodes of 0.363 Mt @ 0.5% Cu, 3.0% Pb and 7% Zn. This resource was regarded to small and the exploration title was abandoned. In 1984, CRA Exploration Pty Ltd, who had outlined the Peak deposit earlier near Cobar, intersected sub-economic mineralisation 4.5 km south of Nymagee. This prospect, which was to become the Hera deposit, passed through the hands of a number of companies, including Pasminco, Triako and CBH Resources until consolidated with the Nymagee deposit in 2009 by the company YTC Resources Ltd that was to become Aurelia Metals Ltd. The Hera deposit had been discovered by Pasminco in 2001, and drilled out by Triako and CBH Resources while the resource at Nymagee was expanded by YTC/Aurelia. Production from the Hera deposit commenced in September 2014 and was fully operational in April 2015. This paragraph is largely drawn from McQueen (2017).
Regional Setting and Geology
The Nymagee, Hera and Federal deposits are located on the Mouramba Shelf, which forms the eastern margin of the ~150 x 50 km, NNW-SSE elongated Cobar Basin, an intra-continental, predominantly extensional basin within the western Lachlan Orogen. The Cobar Basin is mainly filled with siliciclastic turbidites of the Cobar Supergroup. For details of the tectonic and geologic setting see the Geological Setting section of the Cobar Mineral Field record.
The Nymagee area is covered by a thin veneer of Quaternary sediments and is mainly underlain by rocks of the Cobar Supergroup, which on the Mouramba Shelf comprises the Mouramba Group and sections of the overlying to laterally equivalent Amphitheatre Group (MacRae 1987). In contrast, to the north and east of Cobar, the Supergroup is composed of the Kopyje Group of the Kopyje Shelf, and Nurri Group turbidites. In the Nymagee area, the Kopyje Group, which overlies the Ordovician Girilambone Group, interfingers southward and westward into the Mouramba Group which was deposited on a basement of Ordovician turbiditic sedimentary rocks of the Wagga Group (Colquhoun et al., 2005). To the south of Nymagee, the Erimeran Granite, an ~40 km diameter irregular batholith of Late Silurian to earliest Devonian granites, obscures the contact between the basement Girilambone and Wagga groups, and separates the Kopyje and Mouramba shelves which diverge to the SSE (Downes et al., 2016).
The Late Silurian to earliest Devonian (Sherwin 2013) Mouramba Group is dominated by the Burthong Formation which includes a sequence of very fine to medium grained interbedded sandstones and siltstones with minor basal conglomeratic units (MacRae 1987; Pogson 1991). Several volcanic horizons, occurring as polymictic arkosic conglomerate, crystal tuffs and rhyodacite lavas are also mapped (Suppel and Gilligan 1993). Slumping, cross-bedding, graded bedding and soft-sediment deformation is evident in this formation at the Nymagee mine (Paterson 1974), as are current-generated ripple marks and flutes (MacRae 1987), all of which support the interpretation that the Burthong Formation represented an outwash fan to marine shelf sequence (Downes et al., 2016). The Burthong Formation is overlain by, and partially interfingers with the, Roset Sandstone, which is mainly composed of crossbedded lithic quartz to quartz-rich sandstone with a thin pebble conglomerate horizon (Suppel and Gilligan 1993; MacRae 1987).
The Ampitheatre Group conformably overlies the upper parts of the Roset Sandstone and Burthong Formation and represents a deltaic and shallow marine regime, that was deposited within the Cobar Basin and has a predominately turbiditic character. The lower Amphitheatre Group is a folded sequence composed of laminated and massive siltstones with some interlaminated massive sandstones. The two facies may be interbedded. The sandstones are generally fine to medium grained and around 10 to 30 cm thick. Bouma cycles have been observed throughout the sequence (MacRae 1987). This unit outcrops poorly (Glen 1982) and is Lochkovian in age (MacRae 1987).
Mineralisation and Structure
The known mineralisation in the Nymagee corridor occurs as structurally controlled quartz-sulphide matrix breccias grading to massive sulphide. All of the deposits are located from 1 to 3 km to the west of the Rookery Fault, a major NNW-SSE trending regional structure with >300 km of strike length. The deposits are found close to the boundary between the Devonian Lower Amphitheatre Group and the underlying to laterally equivalent Roset Sandstone. Both units have undergone moderate to strong ductile deformation with tight upright folding, coincident with greenschist facies regional metamorphism and a well-developed sub vertical cleavage. Deposits are concentrated in high strain zones and have variable metal ratios, although there is a general tendency for separate Pb+Zn+Ag±Au±Cu and Cu+Ag±Au orebodies. However, there is often a close association between the separate assemblages, with the Pb+Zn lenses lying to the west of the Cu-rich lenses. Formation temperatures are moderate to high - see the Hera description below (Aurelia Metals ASX release 17 July 2018).
The main deposits may be summarised as follows:
Hera
The Hera deposit is hosted by quartz turbidites with interbedded siltstones and shales, which belong to the Mouramba Group. These turbidites exhibit well preserved graded bedding which provide an indication of the younging direction. The siltstones and shales are occasionally discoloured with a green tinge, interpreted to be due to the presence of muscovite and/or chlorite, accompanied by silica alteration, mostly found close to mineralisation. The quartz turbidites are generally relatively brecciated compared to the other lithologies, particularly in the finer grained sections, where the brecciated zones have a vein like distribution. In areas where mineralisation is present the host rocks look highly disturbed and broken, with associated muscovite or chlorite (Page, 2011).
The Pb-Zn-Au mineralisation at Hera is dominated by non-magnetic pyrrhotite, pyrite, galena, sphalerite and chalcopyrite. It is hosted by at least 11 discrete, sediment-hosted, narrow, subparallel lenses. The lenses occur on a fault splay deflected from the NNW-SSE trending Rookery Fault. These lenses strike at 340° and dip steeply to the east, parallel to the axial planes of tight to isoclinal folds (David, 2005, 2008). The mineralisation extends from ~120 m to >600 m below surface. An extensive zone of pervasive silica and chlorite alteration envelopes the mineralisation (Page, 2011). A longitudinal projection shows the overlapping lenses to form an ~45° north plunging envelope that persists for ~500 m down plunge with a width of 250 to 300 m, with additional smaller pods and lenses to its north (Brown et al., 2017).
Mineralisation occurs in the following styles (Brown et al., 2017):
• Disseminations of minor pyrrhotite, sphalerite, pyrite, chalcopyrite and galena developed within foliation planes;
• Vein-like, generally stratabound sphalerite-pyrrhotite-galena-(chalcopyrite) assemblages with hydrothermal quartz that are developed subparallel to bedding.
• Semi-massive to massive sulphides occurring as anastomosing veins forming zinc-, lead- and copper-rich zones. Zinc-rich zones are composed of sphalerite with minor galena, chalcopyrite, pyrrhotite and rare pyrite. Lead-rich zones are dominated by galena and chalcopyrite, with lesser pyrrhotite, pyrite and sphalerite and a high gold content in the form of electrum within galena. Copper-rich zones comprise pyrite, chalcopyrite, pyrrhotite and sphalerite, with subordinate galena and local cubanite (Page, 2011).
Zn is usually more abundant than Pb at Hera with sphalerite the dominant observed sulphide, which is almost always accompanied by galena. The presence of Fe-rich red sphalerite, non-magnetic pyrrhotite and cubanite indicates formation temperatures of between 350 and 400°C. Quartz+K feldspar veins with scheelite and minor associated skarn mineralogy recognised at Hera are taken to suggest a possible magmatic input. The main mineralisation occurs as brittle sulphide matrix breccias with silicification grading to ductile massive sulphides that crosscut both bedding and cleavage. Recent age dating of micas and galena give a Middle Devonian age of ~385 Ma for the Hera deposit (Aurelia Metals ASX release 17 July 2018).
The deposit was discovered beneath a geochemical anomaly with a coincident, weak ground fixed loop electromagnetic (FLEM) anomaly. It does not have a magnetic or gravity response (Brown et al., 2017).
Nymagee
The host rocks found at the Nymagee deposit are a mixture of quartz turbidites interbedded with siltstones and shales of the Mouramba Group, with no recognised marker units. As a consequence, sulphide content is the best available indicator of mineralisation. The turbidites display well preserved graded beds which give an indication as to the younging direction. Generally the beds are steeply dipping with the finer grained mudstone units being overprinted by a pervasive foliation texture defined by the growth of micas including muscovite and biotite. Six lodes have been recognised, the Main lens with 3 footwall lenses in the north and 1 in the south. In 2018, the total resource envelopes had a horizontal strike and down dip extent of 525 m, and a 130 m maximum width in plan, with individual stopes varying from 3 to 30 m wide (Aurelia Metals ASX release 17 July 2018). Historic workings and more recent drilling indicates that the Nymagee lenses plunge at ~60° to the SSE.
Sulphide mineralisation at the Nymagee deposit occurs in both massive and disseminated forms, with the ore lense principally composed of massive sulphides surrounded by a halo of disseminated mineralisation. Chalcopyrite is the principal ore mineral found at the Nymagee deposit, almost always found in association with pyrrhotite and sphalerite. Pyrrhotite, which occurs in both massive and disseminated forms, is also found as veins which occasionally cut across chalcopyrite, with very minor accompanying magnetite. Red sphalerite blebs are found within massive chalcopyrite and pyrrhotite accumulations, whilst black sphalerite is mainly associated with massive sulphides. Cubanite, which is found in association with chalcopyrite at the Nymagee, indicates higher temperatures of hydrothermal mineralisation. The gangue assemblage includes biotite which is overprinted by muscovite, the only mineral to do so, apart from chlorite which may have formed by alteration of biotite. Both biotite and muscovite appear to be alteration minerals related to mineralisation. Amphiboles are found in highly altered rocks with associated chlorite and magnetite (Page, 2011).
Sulphides from Nymagee have a crustal lead-isotope signature with late Silurian lead model ages that range from 428 to 420, averaging 424 Ma. This is consistent an interpretation that lead included in the deposit was sourced from the host late Silurian quartzo-feldspathic sequence i.e. from the Cobar Basin and/or Mouramba Shelf. However, some data are less evolved, suggesting that lead was also sourced from an older reservoir. These data are considered consistent with this lead being sourced from basement-derived material incorporated into the late Silurian sequence and/or also sourced from nearby Ordovician basement units (Downes et al., 2016).
Federation
The Federation deposit occurs within a NE-SW oriented fault corridor that links the NNW-SSE Rookery Fault System to the north, to a parallel structure down the western margin of the Late Silurian Erimeran Granite. The latter intrusion forms the eastern margin of the exposed Mouramba Shelf at Federation. Mineralisation is structurally controlled with several steeply-plunging vein breccia to massive sulphide lenses/shoots with strike lengths of 100 to 150 m, developed within a broad NE-SW striking corridor of low grade lead-zinc mineralisation. These ore shoots are developed over a vertical interval that extends from 25 to 550 m below surface, with intersected widths of >10 m of >10% Zn+Pb. Shallow oxide ore carries high grade gold at depths of < 25 m. The main 100 to 150 m strike length, near vertically plunging ore zone is accompanied by smaller lenses over a total length of >500 m.
Outward from the main breccia zones, there is a general progressive decrease in vein intensity from vein stockwork through fracture-controlled stringer-style mineralisation to selective replacement of pyrrhotite by sphalerite, galena and chalcopyrite in distal wallrocks. The massive sulphide and sulphide breccia base-metal mineralisation is typically zinc rich and is associated with intense cross-cutting black chlorite alteration in the lower parts of the known deposit, and with silica dominant infill in the upper parts. Known high grade gold mineralisation is best developed in steeply plunging shoots within the high-grade zinc and lead lenses. High-grade gold is empirically associated with massive zoned Fe-rich to Fe-poor sphalerite and a distinctly coarse-grained galena infill phase (Aurelia Metals ASX release 8 April 2021; Aurelia Metals ASX release 4 November 2020).
Ore Reserves and Mineral Resources
Ore Reserves and Mineral Resources at 1 July, 2015 (Aurelia Metals Limited Mineral Resource and Ore Reserve Statement, 2015) were:
Hera - pre-mining
Measured + Indicated + Inferred Mineral Resources - 2.506 Mt @ 3.48 g/t Au, 3.65% Pb, 4.76% Zn, 0.15% Cu, 37.21 g/t Ag; including
Probable Ore Reserves - 1.1876 Mt @ 3.59 g/t Au, 2.51% Pb, 3.5% Zn, 0.16% Cu, 15.04 g/t Ag.
Nymagee
Indicated shallow Cu Mineral Resources - 5.147 Mt @ 1.0% Cu, 0.1% Pb, 0.2% Zn, 5 g/t Ag (0.3% Cu cutoff);
Indicated deeper Cu Mineral Resources - 1.984 Mt @ 1.8% Cu, 0.3% Pb, 0.6% Zn, 11 g/t Ag (0.75% Cu cutoff);
Indicated Pb-Zn-Ag lens Mineral Resources - 0.364 Mt @ 0.5% Cu, 4.4% Pb, 7.8% Zn, 41 g/t Ag (5% Pb+Zn cutoff);
Inferred deeper Cu Mineral Resources - 0.601 Mt @ 1.3% Cu, 0.1% Pb, 0.2% Zn, 8 g/t Ag (0.75% Cu cutoff);
TOTAL Mineral Resources - 8.096 Mt @ 1.2% Cu, 0.3% Pb, 0.7% Zn, 9 g/t Ag;
Ore Reserves - Not published
Ore Reserves and Mineral Resources at 30 June, 2018 (Aurelia Metals Limited Mineral Resource and Ore Reserve Statement, 2018) were:
Hera
Measured + Indicated + Inferred Mineral Resources - 2.516 Mt @ 2.44 g/t Au, 2.65% Pb, 3.93% Zn, 25.7 g/t Ag; including
Probable Ore Reserves - 1.117 Mt @ 3.05 g/t Au, 2.84% Pb, 4.36% Zn, 22.7 g/t Ag.
Nymagee
Indicated + Inferred Mineral Resources - 3.780 Mt @ 3.48 g/t Au, 0.84% Pb, 1.63% Zn, 1.31% Cu, 14.3 g/t Ag;
Production target - 1.38 Mt @ 2.2% Cu, 1.58% Pb, 3.15% Zn, 23 g/t Ag.
Remaining Ore Reserves and Mineral Resources at 30 June, 2020 (Aurelia Metals Limited Mineral Resource and Ore Reserve Statement, 2020) were:
Hera
Measured + Indicated + Inferred Mineral Resources - 1.401 Mt @ 1.6 g/t Au, 3.1% Pb, 4.8% Zn, 40 g/t Ag; including
Proved + Probable Ore Reserves - 1.197 Mt @ 1.4 g/t Au, 3.0% Pb, 4.7% Zn, 38 g/t Ag.
Nymagee
Indicated + Inferred Mineral Resources - 1.454 Mt @ 2.2 g/t Au, 0.8% Pb, 1.4% Zn, 18 g/t Ag;
Proved + Probable Ore Reserves - not estimated;
Updated Mineral Resources at February, 2021 (Aurelia Metals ASX release 8 April 2021) were:
Federation
Indicated + Inferred Mineral Resources - 3.49 Mt @ 1.4 g/t Au, 5.5% Pb, 9.8% Zn, 0.3% Cu, 7 g/t Ag; incuding
shallow oxide ore - 0.08 Mt @ 6.0 g/t Au, 2 g/t Ag.
Proved + Probable Ore Reserves - not estimated.
The most recent source geological information used to prepare this decription was dated: 2021.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Brown, R.E., Coffey, J., Hosken, J., Johnston, C. and Lenard, M., 2017 - Cobar district mineral field: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The AusIMM, Melbourne Mono 32, pp. 739-746.
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Downes, P.M., Blevin, P.L., Armstrong, R., Simpson, C.J., Sherwin, L., Tilley, D.B. and Burton, G.R., 2016 - Outcomes of the Nymagee mineral system study - an improved understanding of the timing of events and prospectivity of the central Lachlan Orogen: in Geological Survey of New South Wales, Quarterly Notes, Department of Industry, Resources and Energy, No. 127, 41p.
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McQueen, K., 2017 - Nymagee copper: Birth, death and resurrection?: in Journal of Australasian Mining History, v.15, pp. 99-117.
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Page, D.G., 2011 - Geology of the Hera (Pb-Zn-Au) and Nymagee (Cu) deposits, New South Wales: in University of Wollongong, School of Earth and Environmental Sciences, Bachelor of Science (Honours), Thesis, 212p
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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